1. Field of the Invention
The present invention relates to a photo detector for use in a semiconductor device for infrared remote control reception.
2. Description of the Prior Art
An infrared remote control system widely used in household electrical appliances comprises a transmitter operated in hand and a receiver mounted on an appliance such as a television. In order that the transmitter can be operated even at a distance of 20 m or more from the receiver, the receiver is provided with a high amplification degree. For this reason, the receiver is susceptible to electromagnetic noise coming from outside sources.
Therefore, in a conventional photo detector for use in a receiver, for example, as disclosed in Japanese Laid-open Patent Application No. H3-159180, a shield layer that transmits light is formed on the entire surface of a light receiving portion, or a striped, mesh or radial shield layer that transmits light is formed on the surface of the light receiving portion, and electrode wiring is drawn out of the shield layer and the potential thereof is connected to the ground potential, thereby enhancing noise immunity. Specifically, an n.sup.+ region is diffused into a p-type substrate to form the light receiving portion, the shield layer is formed of a shallow p.sup.+ -type diffusion layer on the n.sup.+ -type region constituting the light receiving portion, and the pn junction forming the light receiving portion is surrounded by the p-type substrate and the shield layer to thereby prevent the influence of external noise.
However, in the conventional photo detector, since the shield layer is formed of the shallow p.sup.+ -type diffusion layer on the n.sup.+ -type region, the p.sup.+ -type diffusion layer serving as the shield layer forms a junction in the n.sup.+ -type region and spontaneous noise is generated from the p.sup.+ n.sup.+ junction, so that the reception distance is shortened. The relationship between the external noise suppressed by the shield layer and the spontaneous noise generated because of the formation of the shield layer is as follows: if the resistance is reduced by increasing the impurity concentration of the p.sup.+ -type diffusion layer of the shield layer in order to effectively reduce the external noise, the spontaneous noise increases that is generated from the p.sup.+ n.sup.+ junction formed of the n.sup.+ -type region and the p.sup.+ -type diffusion layer of the shield layer, so that the reception distance is shortened, and if the impurity concentration is decreased in order to reduce the generation of the spontaneous noise, the shielding function deteriorates to degrade the external noise canceling capability, so that the reception distance is shortened. Thus, according to the structure of the conventional photo detector, it is impossible to sufficiently reduce the noise and it is difficult to increase the reception distance. In order to increase the reception distance, it matters how the spontaneous noise generated due to the shield layer is reduced.
Here, the spontaneous noise will be explained. The spontaneous noise is normal noise generated at a pn junction of a diode, and contributes to the dark current of the light receiving portion. That is, the spontaneous noise is proportional to the dark current generated at the time of reverse bias. The level of the spontaneous noise is high when the p.sup.+ -type region and the n.sup.+ -type region of the p.sup.+ n.sup.+ junction are both high in impurity concentration, and is low when at least one of the regions is low in impurity concentration.
This will be explained by use of an expression. A current represented by the following expression flows through the p-type shield portion and the n-type light receiving portion: EQU I=qA[D.sub.p N.sub.a L.sub.p.sup.-1 .times.exp{-(.phi..sub.0 +V.sub.R)/V.sub.T }+D.sub.n N.sub.d L.sub.n.sup.-1 .times.exp{-(.phi..sub.0 +V.sub.R)/V.sub.T }].times.{exp(-V.sub.R /V.sub.T)-1}
where D.sub.p and D.sub.n are diffusion coefficients, L.sub.p and L.sub.n are diffusion lengths, .phi..sub.0 is the junction potential, V.sub.R is the reverse bias potential, V.sub.T =kT/q, A is the junction area, N.sub.a is the p-type impurity concentration, and N.sub.d is the n-type impurity concentration.
As is apparent from the expression, the spontaneous noise increases as the impurity concentrations increases. The spontaneous noise also depends on the applied reverse bias voltage. The noise becomes a problem when the surface impurity concentration becomes on the order of 10.sup.17 or higher in both the p-type region and the n-type region.